When the link between two LANs comes up, you may well want to make sure
that email that is queued at either end is flushed - sent out to
its destination. This is done by adding the appropriate sendmail
invocation.

Using the bash 'case' statement on an appropriate parameter that pppd
passes into the script accomplishes this. For example, this is
the /etc/ppp/ip-up script I use to handle our WAN links and the
link to my home Ethernet (also handled on the same ppp server).